Senior Conservative lawmaker Amber Rudd has resigned as the UK's Secretary of State for Work and Pensions, accusing Prime Minister Boris Johnson of an assault on democracy.

Her resignation will come as a blow to Johnson, after his brother Jo Johnson resigned on Thursday saying he was "torn between family loyalty and the national interest." Former Conservative MP Phillip Lee also defected to the Liberal Democrats during Prime Minister's Questions on Wednesday.

Rudd posted her letter to the Prime Minister on Twitter Saturday night, saying she could not "stand by as good, loyal moderate Conservatives are expelled" which she called an "assault on decency and democracy."

Johnson, who has been prime minister for six weeks, expelled 21 Conservatives, including Winston Churchill's grandson Nicholas Soames and Ken Clarke, the longest-serving member of Parliament, after they voted to block the Prime Minister's plan to leave the EU without a deal.

Having Rudd join Johnson's Cabinet was considered a coup for the new Prime Minister, and she was considered the most important Remainer in his government.

Speaking on the BBC's Andrew Marr Show on Sunday, Rudd said she had "not seen enough work going into actually trying to get a deal."

"When I asked Number 10 for a summary of what the plan was for actually getting a deal, I was sent a one-page summary," Rudd said.

She said she believes Johnson is "trying to get a deal with the EU," but added that there had been a considerable amount more of resources preparing instead for a no-deal Brexit.

"It's like 80%-90% of government time going into preparing for no-deal and the absence of actually trying to work to get a deal is what has driven 21 of my colleagues to rebel, and I need to join them," Rudd told Marr.

In her letter to the PM on Saturday, Rudd told Johnson that she "no longer" believed "leaving with a deal is the government's main objective."

"This short-sighted culling of my colleagues has stripped the Party of broad-minded and dedicated Conservative MPs. I cannot support this act of political vandalism," she said.